In the manuel they says about: every 3200km but 4000 a 5000 km is fine.
When you change it, recommed to change the oil filter too, and very important clean the oil screen [it's on the bottem of the oil frame]

Here's a crude drawing. When installing this I followed the XR caliper mount suggestions. Note that the small hole must be drilled out. I didn't use the supplied self-tapping screw; instead I used an M5 or M4 bolt + washer + (some half-washer designed to prevent the screw from accidentally un-screwing - don't know the English name for it).

So I'm going to change my oil now, after a question i posted earlier about white residue on my dipstick.
I haven't had the bike for a long time, so I have only changed oil once before. I drained all the oil out, put in half of the new oil, let it run for about 5 minutes and then put in the other half. I also changed the filter. Is this the right procedure?

Also, I read somewhere about a sort of filter inside the frame that needs changing. What is this and where is it?

There are two filters, one in the engine, the other is at the bottom of the front downtube, I check mine every year or so, they don't seem to gather much in the way of debris.
You have to be careful when checking the downtube one as the hose can kink without you realising it if you just tighten the nut without taking care.

I'm looking for a stock NX650 gas tank. Preferably something without a lot of damage. I'll be sandblasting and painting it, so a few scratches is no big deal. Clean on the inside is the most important. Please PM me and we can work something out.